1. Field of Invention
The present invention relates to microelectromechanical system (MEMS) device. More particularly, the present invention relates to a structure of MEMS device and the fabrication process.
2. Description of Related Art
MEMS device has been proposed in various applications. However, the MEMS device usually is staying in an open environment. Since the MEMS structure of the MEMS device is sensitive to the environmental air, the operation in open environment may pick up noise. A hermetic package for MEMS device is then proposed.
A hermetic MEMS package has properties including isolating the MEMS structure from the outside environment. For example, the damping of air and thermal noise can be avoided by forming a vacuum environment for MEMS. In addition, hermetic camber can protects the MEMS device from EM interference and produce a small size MEMS device with high performance and low cost for popularity. The applications for the hermetic MEMS packages include accelerometer, gyroscope, resonator, or RF MEMS components.
For the current hermetic MEMS products, the MEMS structure and sensing or driving circuits are manufactured in separate substrate. The MEMS structures include sensors or actuators. The MEMS sensors are able to sense the physical signal, for example, acceleration and angular velocity, and transfer to the electrical signal and the electrical signal becomes readable via the sensing circuits; while the MEMS actuators are able to create a mechanical motion via the driving circuits. Both sensing and driving circuits are manufactured by well-known CMOS process. FIG. 1 is a cross-sectional view, illustrating conventional hermetic MEMS device. In FIG. 1, the conventional hermetic MEMS device includes a lower substrate 100, a middle substrate 102 and a top substrate 104. The lower substrate 100 is patterned to have an indent space 106 and a CMOS circuit with the connection pad 114 as the sensing IC. The middle substrate 102 is fabricated into a sensing element with the venting holes 110. The top substrate 104 is fabricated like a cap with the indent space 108. The middle substrate 102 is adhered with the bottom substrate 100 and the top substrate 104 by the adhering rings 112. As a result, the indent spaces 106 and 108 with the venting holes 110 form a hermetic chamber.
However, this conventional process has some drawbacks, including high cost, production yield issue due to three elements being packaged into a device, and parasitic effect for the conjunction of MEMS and sensing IC.